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Constants

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A constant is an identifier (name) for a simple value. As the name suggests, that value cannot change during the execution of the script (except for magic constants, which aren't actually constants). A constant is case-sensitive by default. By convention, constant identifiers are always uppercase.

The name of a constant follows the same rules as any label in PHP. A valid constant name starts with a letter or underscore, followed by any number of letters, numbers, or underscores. As a regular expression, it would be expressed thusly: [a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*

Tip

See also the Userland Naming Guide.

Example #1 Valid and invalid constant names

<?php

// Valid constant names
define("FOO",     "something");
define("FOO2",    "something else");
define("FOO_BAR""something more");

// Invalid constant names
define("2FOO",    "something");

// This is valid, but should be avoided:
// PHP may one day provide a magical constant
// that will break your script
define("__FOO__""something"); 

?>

Note: For our purposes here, a letter is a-z, A-Z, and the ASCII characters from 127 through 255 (0x7f-0xff).

Like superglobals, the scope of a constant is global. You can access constants anywhere in your script without regard to scope. For more information on scope, read the manual section on variable scope.

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User Contributed Notes 14 notes

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29
wbcarts at juno dot com
2 years ago
CONSTANTS and PHP Class Definitions

Using "define('MY_VAR', 'default value')" INSIDE a class definition does not work. You have to use the PHP keyword 'const' and initialize it with a scalar value -- boolean, int, float, or string (no array or other object types) -- right away.

<?php

define
('MIN_VALUE', '0.0');   // RIGHT - Works OUTSIDE of a class definition.
define('MAX_VALUE', '1.0');   // RIGHT - Works OUTSIDE of a class definition.

//const MIN_VALUE = 0.0;         WRONG - Works INSIDE of a class definition.
//const MAX_VALUE = 1.0;         WRONG - Works INSIDE of a class definition.

class Constants
{
 
//define('MIN_VALUE', '0.0');  WRONG - Works OUTSIDE of a class definition.
  //define('MAX_VALUE', '1.0');  WRONG - Works OUTSIDE of a class definition.

 
const MIN_VALUE = 0.0;      // RIGHT - Works INSIDE of a class definition.
 
const MAX_VALUE = 1.0;      // RIGHT - Works INSIDE of a class definition.

 
public static function getMinValue()
  {
    return
self::MIN_VALUE;
  }

  public static function
getMaxValue()
  {
    return
self::MAX_VALUE;
  }
}

?>

#Example 1:
You can access these constants DIRECTLY like so:
* type the class name exactly.
* type two (2) colons.
* type the const name exactly.

#Example 2:
Because our class definition provides two (2) static functions, you can also access them like so:
* type the class name exactly.
* type two (2) colons.
* type the function name exactly (with the parentheses).

<?php

#Example 1:
$min = Constants::MIN_VALUE;
$max = Constants::MAX_VALUE;

#Example 2:
$min = Constants::getMinValue();
$max = Constants::getMaxValue();

?>

Once class constants are declared AND initialized, they cannot be set to different values -- that is why there are no setMinValue() and setMaxValue() functions in the class definition -- which means they are READ-ONLY and STATIC (shared by all instances of the class).
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8
php at webflips dot net
4 months ago
It is perfectly valid to use a built-in PHP keyword as a constant name - as long as you the constant() function to retrieve it later:

<?php
define
('echo', 'My constant value');

echo
constant('echo'); // outputs 'My constant value'
?>
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9
katana at katana-inc dot com
12 years ago
Warning, constants used within the heredoc syntax (http://www.php.net/manual/en/language.types.string.php) are not interpreted!

Editor's Note: This is true. PHP has no way of recognizing the constant from any other string of characters within the heredoc block.
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8
storm
9 years ago
An undefined constant evaluates as true when not used correctly. Say for example you had something like this:

settings.php
<?php
// Debug mode
define('DEBUG',false);
?>

test.php
<?php
include('settings.php');

if (
DEBUG) {
  
// echo some sensitive data.
}
?>

If for some reason settings.php doesn't get included and the DEBUG constant is not set, PHP will STILL print the sensitive data. The solution is to evaluate it. Like so:

settings.php
<?php
// Debug mode
define('DEBUG',0);
?>

test.php
<?php
include('settings.php');

if (
DEBUG == 1) {
  
// echo some sensitive data.
}
?>

Now it works correctly.
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4
ewspencer at industrex dot com
11 years ago
I find using the concatenation operator helps disambiguate value assignments with constants. For example, setting constants in a global configuration file:

<?php
define
('LOCATOR',   "/locator");
define('CLASSES',   LOCATOR."/code/classes");
define('FUNCTIONS', LOCATOR."/code/functions");
define('USERDIR',   LOCATOR."/user");
?>

Later, I can use the same convention when invoking a constant's value for static constructs such as require() calls:

<?php
require_once(FUNCTIONS."/database.fnc");
require_once(
FUNCTIONS."/randchar.fnc");
?>

as well as dynamic constructs, typical of value assignment to variables:

<?php
$userid 
= randchar(8,'anc','u');
$usermap = USERDIR."/".$userid.".png";
?>

The above convention works for me, and helps produce self-documenting code.

-- Erich
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3
Andreas R.
7 years ago
If you are looking for predefined constants like
* PHP_OS (to show the operating system, PHP was compiled for; php_uname('s') might be more suitable),
* DIRECTORY_SEPARATOR ("\\" on Win, '/' Linux,...)
* PATH_SEPARATOR (';' on Win, ':' on Linux,...)
they are buried in 'Predefined Constants' under 'List of Reserved Words' in the appendix:
http://www.php.net/manual/en/reserved.constants.php
while the latter two are also mentioned in 'Directory Functions'
http://www.php.net/manual/en/ref.dir.php
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4
anj at aps dot anl dot gov
8 years ago
It is possible to define constants that have the same name as a built-in PHP keyword, although subsequent attempts to actually use these constants will cause a parse error. For example in PHP 5.1.1, this code

    <?php
    define
("PUBLIC", "Hello, world!");
    echo PUBLIC;
   
?>

gives the error

    Parse error: syntax error, unexpected T_PUBLIC in test.php on line 3

This is a problem to be aware of when converting PHP4 applications to PHP5, since that release introduced several new keywords that used to be legal names for constants.
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3
hafenator2000 at yahoo dot com
9 years ago
PHP Modules also define constants.  Make sure to avoid constant name collisions.  There are two ways to do this that I can think of.
First: in your code make sure that the constant name is not already used.  ex. <?php if (! defined("CONSTANT_NAME")) { Define("CONSTANT_NAME","Some Value"); } ?>  This can get messy when you start thinking about collision handling, and the implications of this.
Second: Use some off prepend to all your constant names without exception  ex. <?php Define("SITE_CONSTANT_NAME","Some Value"); ?>

Perhaps the developers or documentation maintainers could recommend a good prepend and ask module writers to avoid that prepend in modules.
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2
tudor at tudorholton dot com
7 years ago
Note that constant name must always be quoted when defined.

e.g.
define('MY_CONST','blah') - correct
define(MY_CONST,'blah') - incorrect

The following error message also indicates this fact:
Notice:  Use of undefined constant MY_CONST - assumed 'MY_CONST' in included_script.php on line 5

Note the error message gives you some incorrect information.   'MY_CONST' (with quotes) doesn't actually exist anywhere in your code.  The error _is_ that you didn't quote the constant when you defined it in the 'assumed' file.
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2
ben at bendodson dot com
6 years ago
I recently found I needed a way of retrieving the value of a constant dynamically - e.g. trying to find the value of FOO_BAR by passing 'FOO_' . $someVariableWithValueBAR.  I came up with the following solution:

<?php

define
('FOO_BAR','It works!');
define('FOO_FOO_BAR','It works again!');

// prints 'It works!'
$changing_variable = 'bar';
echo
constant('FOO_' . strtoupper($changing_variable));

// prints 'It works again!'
$changing_variable = 'foo_bar';
echo
constant('FOO_' . strtoupper($changing_variable));

?>

Note the use of strtoupper() as constants should be defined in uppercase for good practice - feel free to remove if you have constants defined in lowercase or you can set $changing_variable as uppercase.

Might be of some use to someone!
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-1
pdenny at magmic dot com
7 years ago
Note that constants can also be used as default argument values
so the following code:

<?php
  define
('TEST_CONSTANT','Works!');
  function
testThis($var=TEST_CONSTANT) {
      echo
"Passing constants as default values $var";
  }
 
testThis();
?>

will produce :

Passing constants as default values Works!

(I tried this in both PHP 4 and 5)
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-1
kumar at farmdev
10 years ago
before embarking on creating a language system I wanted to see if there was any speed advantage to defining language strings as constants vs. variables or array items.  It is more logical to define language strings as constants but you have more flexibility using variables or arrays in your code (i.e. they can be accessed directly, concatenated, used in quotes, used in heredocs whereas constants can only be accessed directly or concatenated).

Results of the test:
declaring as $Variable is fastest
declaring with define() is second fastest
declaring as $Array['Item'] is slowest

=======================================
the test was done using PHP 4.3.2, Apache 1.3.27, and the ab (apache bench) tool.
100 requests (1 concurrent) were sent to one php file that includes 15 php files each containing 100 unique declarations of a language string.

Example of each declaration ("Variable" numbered 1 - 1500):
<?php
$GLOBALS
['Variable1'] = "A whole lot of text for this variable as if it were a language string containing a whole lot of text";
?>
<?php
define
('Variable1' , "A whole lot of text for this variable as if it were a language string containing a whole lot of text");
?>
<?php
$GLOBALS
['CP_Lang']['Variable1'] = "A whole lot of text for this variable as if it were a language string containing a whole lot of text";
?>

Here are the exact averages of each ab run of 100 requests (averages based on 6 runs):
variable (24.956 secs)
constant (25.426 secs)
array (28.141)

(not huge differences but good to know that using variables won't take a huge performance hit)
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-4
kencomer at NOSPAM dot kencomer dot com
8 years ago
Being a belt and suspenders person, when I use a constant to do flow control (i.e., using constants to determine which version of a section of the program should be used), I always use something like:

if ( defined('DEBUG') && TRUE===DEBUG )

If you accidentally use DEBUG somewhere before it is defined, PHP will create a new constant called DEBUG with the value 'DEBUG'. Adding the second comparison will prevent the expression from being TRUE when you did not intentionally create the constant. For the constant DEBUG, this would rarely be a problem, but if you had (e.g.) a constant used to determine whether a function was created using case-sensitive comparisons, an accidental creation of the constant IGNORE_CASE having the value 'IGNORE_CASE' could drive you up the wall trying to find out what went wrong, particularly if you had warnings turned off.

In almost all code I write, I put this function definition in my configuration section:

<?php
if (!function_exists("debug_print")) {
  if (
defined('DEBUG') && TRUE===DEBUG ) {
    function
debug_print($string,$flag=NULL) {
     
/* if second argument is absent or TRUE, print */
     
if ( !(FALSE===$flag) )
        print
'DEBUG: '.$string . "\n";
    }
  } else {
    function
debug_print($string,$flag=NULL) {
    }
  }
}
?>

Then, in my code, I'll sprinkle liberal doses of debug code like :

<?php
define
("DEBUG_TRACK_EXAMPLE_CREATION",FALSE);
class
Example extends Something {
 
__construct($whatever) {
   
debug_print( "new instance of Example created with '$whatever'\n",DEBUG_TRACK_EXAMPLE_CREATION);
  }
}
?>

and :

<?php
debug_print
("finished init.\n");
?>

In the first case, I would not want to see that message every time I went into DEBUG mode, so I made it a special case. The second case is always printed in DEBUG mode. If I decide to turn everything on, special cases and all, all I have to do is comment out the "if" line in debug_print() and presto magicko! It costs a little and gains a lot.

As another belt-and-suspenders aside, notice that, unlike most people, I put the language constant (e.g.,TRUE, "string", etc.) on the left side of the comparison. By doing that, you can never accidentally do something like
  if ( $hard_to_find_error="here" )

because you always write it as
  if ( "here"==$no_error )

or, if you got it wrong,
  if ( "here"=$easy_to_find_parse_error )
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-8
martin at larsen dot dk
8 years ago
I find variables much more flexible than constants because variables can be used inside quotes and heredocs etc. Especially for language systems, this is nice.

As stated in one of the previous notes, there is no speed penalty by using variables. However, one issue is that you risc name collision with existing variables. When implementing a language system I simply found that adding a prefix to all the variables was the way to go, for example:

$LNG_myvar1 = "my value";

That is easier and performs faster than using arrays like

$LNG['myvar'] = "my value";

As a final note, implementing a new superglobal in PHP would make using constants much more beneficial. Then it could be used in qoutes like this:

"The constant myconst has the value $CONSTANTS[myconst] !"
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